1. Field of the Invention
The present invention relates to a projection optical apparatus including an imaging optical system and more particularly to the detection of variations in the optical characteristics of the imaging optical system.
2. Description of the Prior Art
With projection optical apparatus, e.g., reduction projection-type exposure apparatus or steppers heretofore used as lithographic apparatus for the manufacturing process of very-large-scale integrated circuits of VLSI circuits, the recent demand for the increase of productivity has increased the throughout and the exposure light passed through the projection optical system within a unit time has been increased correspondingly. The energy of the exposure light is partly absorbed and converted to heat by the lenses of the projection optical system, thus causing variations in the refractive index, shape, etc., of the lenses. If such variations of the optical system due to the heat are greater than given values, the imaging characteristics such as the focal position and imaging magnification are affected to degrees that cannot be ignored.